Switch lowering: extract jump tables and bit tests before building binary tree (PR22262)

This is a major rewrite of the SelectionDAG switch lowering. The previous code

would lower switches as a binary tre, discovering clusters of cases

suitable for lowering by jump tables or bit tests as it went along. To increase

the likelihood of finding jump tables, the binary tree pivot was selected to

maximize case density on both sides of the pivot.

By not selecting the pivot in the middle, the binary trees would not always

be balanced, leading to performance problems in the generated code.

This patch rewrites the lowering to search for clusters of cases

suitable for jump tables or bit tests first, and then builds the binary

tree around those clusters. This way, the binary tree will always be balanced.

This has the added benefit of decoupling the different aspects of the lowering:

tree building and jump table or bit tests finding are now easier to tweak

separately.

For example, this will enable us to balance the tree based on profile info

in the future.

The algorithm for finding jump tables is O(n^2), whereas the previous algorithm

was O(n log n) for common cases, and quadratic only in the worst-case. This

doesn't seem to be major problem in practice, e.g. compiling a file consisting

of a 10k-case switch was only 30% slower, and such large switches should be rare

in practice. Compiling e.g. gcc.c showed no compile-time difference. If this

does turn out to be a problem, we could limit the search space of the algorithm.

This commit also disables all optimizations during switch lowering in -O0.

Differential Revision: http://reviews.llvm.org/D8649